Use the Pixelate effect to apply a mosaic effect to an image (and its matte) based on the matte you supply. Pixelation makes the image look like it is made up of square tiles.
There are two input ports on the Pixelate module:
|•||The left port receives the matte|
|•||The right port receives the image|
Use the Pixelate editor to control the amount (Factor) the image is pixelated. You can attach the Factor value to a function curve to change the value over time.